The present invention relates to an image processing apparatus, and specifically to a technology for efficiently compressing image data when the image data is stored.
Recently, facsimiles or digital copiers have semiconductor memories or magnetic recording medium for recording image data. Image data read from original documents is not only recorded once, but can be used many times. When image data is recorded in a semiconductor memory or a magnetic recording medium, it is preferable to compress image data as much as possible. The reason for this data compression is that cost for memory, which can be saved by data compression, greatly exceeds the cost for providing additional compression function, for enormous amounts of image data.
There are two data compression methods, a reversible compression method and an irreversible compression method. In the irreversible compression method, a high compression ratio can be obtained if some deterioration of the image is acceptable. However, when a hard copy is used, as in the case of a digital copier or the like, the reversible compression is preferable because characters are mixed with photographs. In this connection, many algorithm have been considered for compressing image data, and an LSI for compressing and decompressing image data has been achieved. For example, a MH (Modified Huffman) method, Huffman algorithm for coding, such as JPEG, and an LZ (Lempel-Ziv) method, or the like, which are used in a facsimile, are realized.
In this connection, because high speed and a large amount of data are generated in the digital copier, or the like, it is necessary for the copier, or the like, to correspond to the compression processing speed of image data, and the amount of the data.
For example, in the digital copier, when the copy speed for A4-sized sheets is 60 pages per minute, the pixel density is 16 dots/mm, and the data width is composed of 8 bits, then, the data rate is about 16 Mbytes/sec. In this case, when an LSI for compression having a compression processing speed of a compression section of 2.0 Mbytes/sec, is used, it is necessary that the data is parallelly processed using 8 LSIs for compression.
Concerning the amount of data, it is preferable to efficiently compress a large amount of image data generated in the copier, or the like, as much as possible to save memory space.